Well, if it's backup photo folder and is not created by you can't be deleted by you there are some system files that are automatically generated by the Android system but, if you think that it is created by you and now you are unable to do that I think that folder might be open by the android system in the back ground that may be the reason why you are unable to delete that folder. Well in this case shut down your device and restart it after a while and upon the restart try delete the file again it should get removed but if still can't be deleted then it's definitely a system file and it can't be removed.